It may be worth a telephone call to the therapist to ask more about the exercises. Perhaps they were meant to be used short-term until your son was able to get back to his regular exercise program. If that's the case, then he's free to go about his business.

If the exercises were indeed preventive, then he may need a better understanding of what the exercises are supposed to do and how they will help prevent future injuries.

This age group (13 to 18 years old) are often non-compliant with a home exercise program. If the exercises are important, it may be helpful to involve his track coach. Perhaps it's something that could benefit the entire team and could be incorporated into their regular warm-up exercises.